This paper describes the circuits used in a decimal arithmetic unit which utilizes ferrite magnetic elements and copper conductors only. The arithmetic operations of addition, subtraction, and multiplication are performed with a product and sum capacity of three decimal digits. The sole logical building block of this system is a two-input inclusive-OR module with a fan-out capability of three with any desired logical positive and negative combination. The system involves the use of some 325 modules, each of which contains two magnetic multiaperture devices (MAD's). This paper gives a complete description of the circuit and physical arrangement of the machine. The system is controlled from a manual keyboard, and readout from the machine is via incandescent lamps controlled directly from the MAD elements, no intermediate elements being required. The "worst case" drive-pulse amplitude range for the completed machine, varying all clock pulses simultaneously, is ± 10 per cent.